Bilateral asymmetry of respiratory acoustic transmission

Sonic noise transmission from the mouth to six sites on the posterior chest wall is measured in 11 healthy adult male subjects at resting lung volume. The measurement sites are over the upper, middle and lower lung fields and are symmetric about the spine. The ratios of transmitted sound power to analogous sites over the right (R) and left (L) lung fields are estimated over three frequency bands: 100-600 Hz (low), 600-1100 Hz (mid) and 1100-1600 Hz (high). A R-L dominance in transmission is measured at low frequencies, with a statistically significant difference observed at the upper site. No significant asymmetry is observed in any measurement site at mid or high frequencies. A theoretical model of sound transmission that includes the asymmetrical anatomy of the mediastinal structures is in agreement with the observed asymmetry at low frequencies. These findings suggest that the pathway of the majority of sound transmission from the trachea to the chest wall changes from a more radial to airway-borne route over the measured frequency range.